The common method presently used for fuel measurement in aircraft tanks employs a capacitance measuring technique in which two plates are mounted within the fuel tank. The fuel, being a dielectric between the plates, creates a varying capacitance as the quantity varies. To measure the capacitance, electrical energy is required to be applied to the plates which necessitates the use of electrical wiring into the tank. If a fault occurs within the capacitor plate assembly, or in the electrical wiring either inside or outside the tank from either the capacitance measuring system or other non-associated systems, such as a short circuit, a resulting high current could possibly flow in the short. This short can be produced by foreign material as small as a single filament of steel wool or by abrasion and shorting in the electrical wiring. These conditions within the tank result in a potentially hazardous, explosive condition. Results from fault analysis have shown that several amperes can flow through the shorted element if, for example, a standard laboratory multimeter such as a Simpson Model 260-5P is used to check the capacitance element and the multimeter is not properly connected. Moreover, corrective maintenance of the capacitance measuring system requires tank defueling, tank opening, tank purging and the entry of personnel in this hazardous environment together with the use of electrical troubleshooting apparatus. Several on-board aircraft fires are said to have been associated with the capacitance probe fuel quantity measurement system.
In applications involving less volatile liquids such as oil and the like mechanical floats with potentiometer movement are sometimes employed. Such mechanisms are unacceptable in the more dynamic cases of aircraft because of their unreliability due to continual motion and a resulting short life. As an alternate, hot wire devices have been proposed. In this system electrical energy is used to heat a fine wire partially submerged in the oil and the resistance is measured.